1. Field of the Invention
The present invention relates to a method for inspecting the dimensions of a male or female tapered thread located on the periphery of a male or female tubular element itself located at the end of a metal pipe, more particularly to a process for inspecting the dimensions of the pitch diameter of such a male or female tapered thread in a given cross-sectional plane and inspection devices for carrying out the method.
The term xe2x80x9cpitch diameterxe2x80x9d of a thread means the diameter taken on the flank of the male thread at the thread half-height. The female thread is defined with respect to the male thread.
For a tapered thread, the nominal value of the pitch diameter must be defined in a given cross-sectional plane.
Threaded connections for metal pipes are known which are in particular used to constitute drilling, production tubing or casing pipes for hydrocarbon wells or mine wells, the connection being made between a male element and a female element located at the pipe end and each respectively provided with a male and female tapered thread located respectively on the external or internal periphery of the male or female element.
The term xe2x80x9cpipexe2x80x9d as used here means any type of pipe, not simply a long pipe but also a short tubular element forming, for example, a coupling and enabling two long pipes to be associated together.
Specification API 5CT from the American Petroleum Institute (API), which constitutes a world-wide standard in the hydrocarbon extraction industry, specifies pipes coupled using such threaded connections comprising tapered threaded portions with triangular, round or trapezoidal threads.
Specification API 5B, also from the API, specifies the corresponding thread and a method for inspecting it.
Specification API 5B specifically mentions, for each pipe dimension, the value of the thread nominal pitch diameter in a cross-sectional plane located at the end of the perfect male threads on the pipe body side, male threads exceeding this plane having an incomplete height and fading out.
In the remainder of the present document, this plane is termed the xe2x80x9creference plane of the pitch diameterxe2x80x9d, abbreviated to xe2x80x9creference planexe2x80x9d. The term xe2x80x9cfirst threadxe2x80x9d relates to the side of the threaded portion directed towards the free end of the corresponding male or female element; the term xe2x80x9clast threadxe2x80x9d refers to the side of the threaded portion oppositely directed to the free end of the corresponding element.
The last perfect male thread is thus located in the reference plane while the last male thread corresponds to the end of the threaded portion on the pipe body side.
Threads produced in accordance with API 5B must be inspected by hand tightening of gauges such as ring gauges comprising an internal thread in the case of inspecting male threads or plug gauges comprising an external thread in the case of inspecting female threads.
The relative axial position when the gauge has been screwed on is checked with respect to the inspected thread and specification API 5B defines a value and a tolerance for this relative axial position.
The inspection method specified by API 5B has advantages, namely simple and rapid global inspection of the thread, but it also has a number of economical and technical disadvantages.
Firstly, for each thread diameter to be inspected, the hard gauge inspection method requires sets of gauges for different levels, namely master gauges and secondary or working gauges, the working gauges having to be discarded when the wear on them exceeds a critical value.
This means that an extremely large number of high precision gauges have to be made and they have to be managed depending on their state of wear; thus costs are high.
That thread inspection method provides a global result which depends on a number of parameters including the pitch diameter and also the taper and ovality, which parameters interact and thus do not facilitate fine interpretation of the inspection results.
Thus when inspecting a male thread, if the taper of the male thread to be inspected is less than that of the gauge, the first roots of the male threads are in contact with the gauge threads while the last roots of the male threads have a radial clearance with respect to the corresponding threads of the gauge. In contrast, if the taper of the male thread to be inspected is higher than that of the gauge, the roots of the last male threads are in contact with the threads of the gauge but not the roots of the first threads.
In both cases, the pitch diameter of the male thread in the corresponding plane on the gauge in the reference plane is less than the nominal pitch diameter but, in addition in the second case, the pitch diameter at the first male threads is not accurately known.
Similarly, in the case of inspecting a female thread, when the taper of the female thread to be inspected is less than or greater than that of the gauge, the pitch diameter of the female thread in the corresponding plane on the gauge to the reference plane is higher than the nominal pitch diameter but, if is its lower, the pitch diameter at the first male threads is not accurately known.
The manufacturers of particular threaded connections such as the connections described in European patent EP-A-0 488 912, which are known to have service performances which are superior to those of API connections, have been forced to use inspection methods similar to those specified for API connections because of international recognition and imposition of API specifications.
The cost of implementing those methods is considerable; the manufacturer must have available complete sets of gauges for itself and for sub-contractors.
Thread inspection methods have thus been developed which do not use hard gauges but which carry out a direct determination of the pitch diameter in the reference plane or at another set position.
United States patent U.S. Pat. No. 4,524,524 describes a method and a device for direct inspection of the pitch diameter of a horizontally disposed male or female thread in which:
The device comprises an upper contact surface and a lower contact surface located in a vertical plane at an adjustable horizontal distance from a vertical bearing surface;
The vertical distance between the upper and lower contact surfaces are adjusted to a pre-determined value;
The device is placed such that the vertical bearing surface bears against the end of the element the thread of which is to be inspected and such that the two contact surfaces are in contact with the thread crests at diametrically opposed points on the thread;
The difference in distance between these two contact surfaces is measured with respect to the pre-determined value using a comparator, for example, which has been zeroed to the predetermined value.
The pre-determined value in this case corresponds to the nominal value of the diameter between the thread crests and thus to the nominal value of the pitch diameter increased or reduced by the height of one thread depending on whether a male or female thread is being inspected.
The instructions for the device sold by the proprietor of U.S. Pat. No. 4,524,524 defines:
a) the required relation for establishing the link between the tolerance in the pitch diameter (xcex94D) when using the inspection method using the measurement of the pitch diameter and the tolerance in the axial position (xcex94S) of the hard gauge as defined by specification API 5B:
xcex94D=xcex94Sxc2x7TTnom/100
xe2x80x83TTnom being the thread nominal taper in % with respect to the diameter;
b) fine corrections for accounting for the influence of the geometry of the contact surfaces of the inspection device on the pre-determined value of the vertical distance between the contact surfaces;
c) the method for determining the pre-determined value of the vertical distance between the contact surfaces of the inspection device to account for location of the measuring plane out of the reference plane. Thus a quantity equal to: Lxc2x7TTnom/100 is subtracted from the nominal pitch diameter;
xe2x80x83L being the axial distance between the measuring plane and the reference plane and TTnom being the nominal value of the taper of the thread expressed as a % with respect to the diameter.
The instructions for that device do not, however, attach any particular importance to carrying out the dimensional inspection of the pitch diameter in a measuring plane other than the reference plane.
In a first aspect, the present invention has sought to develop a method for inspecting a male or female tapered thread which is more particularly adapted to connections with a high degree of tightness the male and female elements of which comprise at least one sealing means, which does not use a hard gauge but which can produce and even guarantee the same performance as the hard gauge inspection method.
We have thus sought to use a method in which the pitch diameter is inspected in a given measuring plane using a plane diameter measuring device.
In the remainder of the present document, the term xe2x80x9cplane diameter measuring devicexe2x80x9d, abbreviated to xe2x80x9cmeasuring devicexe2x80x9d, means a device similar or equivalent to that described in U.S. Pat. No. 4,524,524 which can be used to measure a diameter in a given cross-sectional plane of an object to be inspected and which comprises:
a transverse bearing surface;
at least two contact surfaces defined with respect to the measuring plane and a transverse distance from each other and at an adjustable axial distance from the bearing surface;
and a means for measuring the diameter of the circle located in a transverse measuring plane tangential to the contact surfaces at a given axial distance from the bearing surface.
We have also sought to measure the pitch diameter in the cross-sectional plane where it is the most important to carry out the measurement considering the expected sealing characteristics of the connections to be inspected.
One disadvantage of using a plane diameter measuring device is that the pitch diameter to be inspected is only accurately known close to the measuring plane; the value of the pitch diameter in a plane relatively far from the measuring plane is highly uncertain because of the manufacturing tolerances on the taper of the thread to be inspected.
The method of the invention for inspecting a male or female tapered thread located on the external or internal periphery of a male or female tubular element itself located at the end of a metal pipe and comprising at least one sealing means is a method in which the pitch diameter of the thread is inspected in a given measuring plane located at a distance L from the reference plane of the drawing using a plane diameter measuring device provided with a diameter measuring means.
The term xe2x80x9csealing meansxe2x80x9d as used in the present document defines a means such as, for example, a sealing surface, a transverse bearing surface or an equivalent means.
The male sealing means is/are located close to the free end of the male element while the female sealing means is/are located on the female element so as to co-operate with that/those close to the free end of the male element to which the female element is intended to be coupled.
The method comprises the following steps:
a) a step for adjusting, on said plane diameter measuring device, the distance between the bearing surface and the measuring plane, this distance being a function of the distance between the measuring plane and the reference plane;
b) a step for adjusting said measuring means to a value of the transverse distance between the contact surfaces using a setting block the characteristic dimension of which is defined with respect to the estimated value of the pitch diameter in the measuring plane. Depending on whether the male or female thread is to be inspected, the characteristic dimension is respectively higher or lower than the estimated value of the pitch diameter in the measuring plane by a quantity h; this quantity h is equal to the sum of the height of one thread and a known geometrical correction factor;
c) a step for measuring the diameter between the thread crests of the thread in the selected measuring plane, said measuring device being brought into abutment by its bearing surface against the free end of the element under consideration;
d) a step for comparing the measured diameter between the thread crests with respect to the admissible limits.
The measuring plane for the pitch diameter of the male thread is a plane located between the reference plane and the first perfect male thread. The choice of the measuring plane must of course enable the contact surfaces of the measuring device to be able to be applied over a sufficient length of perfect thread crests.
The measuring plane for the pitch diameter of the male thread is preferably the plane located substantially axially half way between the reference plane and that corresponding to the first perfect male thread.
The measuring plane for the pitch diameter of the tapered female thread is the plane of the drawing which coincides with the measuring plane for the pitch diameter of the male thread when the two male and female threads are connected in the drawing.
In a variation, if the plane of the drawing coinciding with the measuring plane for the pitch diameter of the male thread does not fall into a zone of perfect female threads, the measuring plane for the pitch diameter of the female tapered thread is the cross-sectional plane located in the zone of perfect female threads which is the closest to said coinciding plane.
The estimated value D1e of the pitch diameter of the male thread in the measuring plane is obtained using the following formulae, all of the taper values in the remainder of the present document being referred to the diameter and expressed as a %:
D1e=Dnomxe2x88x92L1xc2x7TTrep1/100                              TT          rep1                =                  xe2x80x83                ⁢                              TT            nom                    +                                    K1              ·              Δ                        ⁢                          xe2x80x83                        ⁢            TT1                    +                                                                      K1                  ·                  σ                                ⁢                                  xe2x80x83                                ⁢                1                                                              2                  ⁢                  π                                                      ·                          exp              ⁡                              [                                                      -                                          1                      2                                                        ·                                                            (                                              Δ                        ⁢                                                  xe2x80x83                                                ⁢                                                  TT1                          /                          σ                                                ⁢                                                  xe2x80x83                                                ⁢                        1                                            )                                        2                                                  ]                                                                                                  -                          xe2x80x83                        ⁢            K1                    ·          Δ                ⁢                  xe2x80x83                ⁢                  TT1          ·                      g            ⁡                          (                                                -                  Δ                                ⁢                                  xe2x80x83                                ⁢                                  TT1                  /                  σ                                ⁢                                  xe2x80x83                                ⁢                1                            )                                          
The estimated value D2e of the pitch diameter of the female thread in the measuring plane is obtained by the following formulae:
D2e=Dnomxe2x88x92L2xc2x7TTrep2/100                              TT          rep2                =                  xe2x80x83                ⁢                              TT            nom                    +                                    K2              ·              Δ                        ⁢                          xe2x80x83                        ⁢            TT2                    -                                                    K2                ·                σ2                                                              2                  ⁢                  π                                                      ·                          exp              ⁡                              [                                                      -                                          1                      2                                                        ·                                                            (                                              Δ                        ⁢                                                  xe2x80x83                                                ⁢                                                  TT2                          /                          σ                                                ⁢                                                  xe2x80x83                                                ⁢                        2                                            )                                        2                                                  ]                                                                                                  -                          xe2x80x83                        ⁢            K2                    ·          Δ                ⁢                  xe2x80x83                ⁢                  TT2          ·                      g            (                          Δ              ⁢                              xe2x80x83                            ⁢                              TT2                /                                  σ                  ⁢                  2                                                      )                              
Indices 1 and 2 in the equations relate to the male and female threads respectively.
Dnom is the nominal value of the pitch diameter in the reference plane;
TTrep is the plot value for the taper;
L is the distance between the measuring plane and the reference plane and is positive when the measuring plane is located on the lower diameter side with respect to the reference plane;
TTnom is the nominal value of the thread taper;
xcex94TT is the algebraic value of the difference (TTavxe2x88x92TTnom);
TTmin, TTmax and TTav are respectively the minimum, maximum and mean values of the taper of the threads produced;
"sgr" is the standard deviation of the distribution of the taper values produced;
K1 is the ratio of the length of the male thread to the distance between the reference plane (P0) and the first perfect male thread and K2 is the ratio of the length of the female thread to the distance between the reference plane (P0) and the last perfect female thread;
g(x) is the value of the reduced centred normal distribution for the value x of the variable.
The plot value of the taper TTrep defined by its equation corresponds to the taper of a fictitious cone the large diameter of which equal to the nominal value of the pitch diameter is situated in the reference plane and the small diameter of which equal to the mean value of the pitch diameters of the thread being inspected by means of hard gauges is situated in the plane located at the end of the threads of the inspected thread on the side of at least one sealing means.
The present invention solves the problem concerning the place where it is the most important to measure the pitch diameter of the thread to be inspected to optimise the sealing characteristics of the threaded connection.
The inventors have become aware when developing the invention that too large a diametrical thread interference in the zone of the first perfect male threads has a deleterious effect on the sealing of the connection, in particular when a metallic sealing surface is provided between the male thread and the end of the male element to radially interfere with a metallic sealing surface located on the female element.
The diametrical interference between mated points on two surfaces of revolution which radially interfere is generally defined as the difference in the cross-sectional diameter of the surface at these points, measured before connection and being positive when the two surfaces, once connected, exert a contact pressure between the mated points. This definition is applicable to both the interfering threads and to sealing surfaces.
To estimate the value of the diametrical thread interference close to the first perfect male threads, it is necessary to measure the pitch diameter closer to this zone rather than near the last perfect male threads in order not to introduce a large uncertainty due to variability in the taper: such an aim is taken into account in the present invention.
The present invention can also guarantee that the mean value of the pitch diameters of the thread inspected by the method of the present invention in the plane of threads close to at least one sealing means will be the same as when carrying out the inspection using the method of the present invention or using hard gauges as specified in API 5B, even if the measuring plane is not located quite at the end of the thread.
The mean value of the taper of the female thread is preferably less than the mean value of the taper of the male thread associated therewith.
Such a distinction encourages the production of connections where the thread interference is reduced close to at least one sealing means, the value of the diametrical interference of the thread at this level being optimally appreciated by the inspection method of the invention.
The mean value of the taper of the male thread is preferably higher than the nominal value.
In a variation, the mean value of the taper of the female thread is less than the nominal value.
The admissible minimum and maximum values for the diameter measured between the thread crests can be directly defined from the admissible minimum and maximum values for the pitch diameter in the considered measuring plane increased or decreased by the height of one thread depending on whether a male or female thread is to be inspected.
The admissible minimum and maximum values for the pitch diameter can be defined directly from tolerances on the estimated value of the pitch diameter D1e or D2e or by replacing the value of the distance between the reference plane and the measuring plane by the admissible minimum and maximum values of this distance in the formula giving the estimated value of the pitch diameter in the measuring plane.
During the step for adjusting the measuring means, the measuring means is preferably zeroed then during the measuring step, a difference is measured with respect to zero and finally, during the comparison step, the difference is compared with a tolerance range.
Advantageously, when the measuring device has two contact surfaces, the inspection method is carried out four times in the same measuring plane by turning the measuring device or the thread to be inspected by an eighth of a turn between each measurement about the axis of the connection, the diameter between the thread crests in the measuring plane under consideration being taken to be equal to the mean value of four measurements.
In a variation, when the measuring device has three contact surfaces located at 120xc2x0 to each other, the inspection method is carried out three times, turning the measuring device or the thread by 40xc2x0, i.e., a ninth of a turn, about the connection axis between each measurement.
In a second aspect of the invention, the inventor has provided a plane diameter measuring device which enables the method of the invention to be carried out rapidly and thus economically.
When the measuring device has two contact surfaces, the device must be pivoted about one of the two contact surfaces to detect the point of the thread to be inspected which is diametrically opposite the point where the pivot contact surface is applied, the diameter between the thread crests corresponding to the maximum in the measurement made during this pivoting action. The measuring device thus comprises a means for automatically acquiring this maximum.
The invention also seeks to enable the inspection method of the invention to react rapidly in the event of a drift of the production from specifications.
To this end, said measuring device comprises a means for carrying out statistical calculations on the acquired values.
In a third aspect, the invention provides a setting block used to adjust the pre-determined value of the distance between the contact surfaces in the inspection method of the first aspect of the invention.
When inspecting male tapered threads, the setting block used with a measuring device with two contact surfaces is, in accordance with this third aspect, in the form of a truncated wedge comprising:
a transverse end surface; and
two flat faces with a substantially longitudinal orientation, inclined symmetrically with respect to the transverse end surface and converging towards it;
the angle between said flat faces being equal to 2. arc tan (TTav1/2); and
the transverse distance between said inclined flat faces being equal to (D1e+h) to the longitudinal distance LA from the end surface where h is the quantity defined previously.
In a variation, the setting block used with a measuring device with three contact surfaces is in the form of a truncated cone and comprises a transverse end surface at the cone vertex side and a peripheral tapered surface with a taper of TTav1, the diameter of the tapered surface at a distance LA from the transverse end surface being equal to (D1e+h).
The setting block can also comprise, at the end of its inclined flat faces or of its tapered peripheral surface at the side of the end surface, a portion with a different taper or slope which reproduces the profile of the sealing means of the male element. Such a setting block enables a second plane diameter measuring device to be adjusted in particular to inspect the diameter of the sealing surface.
When inspecting female tapered threads, the block used with a measuring device with two contact surfaces in this third aspect has a transverse end surface and an internal space delimited by two flat surfaces of the block, with a substantially longitudinal orientation, symmetrically inclined with respect to said end surface and converging towards the back of the internal space, the angle between said inclined flat faces being equal to 2.arc tan(TTav2/2) and the transverse distance between said inclined flat faces being equal to (D2exe2x88x92h) at a longitudinal distance LB from the end surface, where h is the quantity defined above.
In a variation, to inspect female tapered threads, the setting block used with a measuring device with three contact surfaces comprises a transverse end surface and an internal space delimited by a tapered peripheral surface with a longitudinal axis and a taper of TTav2, the vertex of which is directed towards the side opposite to the transverse end surface and in which the diameter at distance LB from the transverse end surface is equal to (D2exe2x88x92h).
The setting block for inspecting female tapered threads can also comprise, at the end of its inclined flat surfaces or its tapered peripheral surface at the side opposite to its transverse end surface, a portion with a different taper or slope which reproduces the profile of at least one sealing means on the female element. Such a setting block enables a second plane diameter measuring device to be adjusted to inspect the diameter of the sealing surface.